Platelets are cell-like anucleated fragments, found in the blood of all mammals, which participate in blood coagulation. Fibrinogen is a glycoprotein present as a normal component of blood plasma. Fibrinogen participates in platelet aggregation and fibrin formation in the blood clotting mechanism. Platelets are deposited at sites of vascular injury where multiple physiological agonists act to initiate platelet aggregation culminating in the formation of a platelet plug to minimize blood loss.
When a blood vessel is damaged either acutely by damage such as wounding or clinical interventions like angioplasty, or more chronically by the pathophysiological processes of arteriosclerosis, platelets are activated to adhere to the disrupted surface and to each other. This activation, adherence and aggregation may lead to occlusive thrombus formation in the lumen of the blood vessel, preventing blood flow and resulting in acute thrombotic syndromes.
Platelet membrane receptors are essential in the process of platelet adhesion and aggregation. Interaction of fibrinogen with a receptor (fibrinogen receptor) on the platelet membrane, a protein complex known as glycoprotein IIb/lIIa (i.e., GP IIb/IIIa, also known as integrin .alpha..sub.IIb .beta..sub.3) is known to be essential for normal platelet function. More specifically, the final obligatory step in platelet aggregation is the binding of fibrinogen to an activated membrane-bound glycoprotein complex IIb/IIIa. During this platelet activation process, GP IIb/IIIa undergoes an ill-defined change in the spatial orientation of extracellular domains resulting in the exposure of the occult binding sites for fibrinogen. The biochemical processes that underlay conversion of GP IIb/IIIa to the form competent to bind fibrinogen are unknown, however, these conformational changes are paralleled by the exposure of neoepitopes on the GP IIb/IIIa molecule that may be detected with antibodies. The ability of GP IIb/IIIa to undergo this conformational change is an intrinsic property of the protein complex because the activation dependent epitopes may be induced in solubilized and purified GP IIb/IIIa by antibodies and by small molecules.
Thrombocytopenia is a platelet disorder characterized by a decrease in the number of blood platelets (e.g., thrombocytopenia is typically defined as present when the platelet count is below 100,000/.mu.l). Thrombocytopenia may stem from failure of platelet production, sequestration of platelets in the spleen or liver, increased platelet destruction or utilization, or dilution of circulating platelets. Regardless of the cause, severe thrombocytopenia often results in a typical pattern of bleeding. When platelet counts are in the range of 40,000 to 60,000/.mu.l, there may be post traumatic bleeding; and at counts of 20,000/.mu.l, spontaneous hemorrhage is a strong possibility.
More than 80 drugs have been implicated or established as inducers of thrombocytopenia. When the thrombocytopenia is drug-induced, it can have a nonimmune or an immune pathogenesis. Immunologic reactions that terminate in platelet destruction occur when the drug (or one of its derivatives or metabolites) acts as a hapten and forms a complex with the plasma protein. This complex has antigenic properties and induces production of high affinity antibodies. The drug-antibody complex is capable of and sometimes binds to the platelet surface resulting in subsequent removal of the antibody-coated platelets from circulation giving rise to thrombocytopenia.
An alternate mechanism involves direct binding of the drug to the platelet surface, which then exposes antigenic sites to which the antibody binds. The antibody might also bind directly to platelet-bound drug or bind to a drug-platelet membrane complex.
The identification of agents which act as inhibitors of platelet aggregation and which bind selectively to the activated form of platelets and the activated, isolated GP IIb/IIIa is desirable since this type of selective antiplatelet agent would minimize the potential for drug-induced thrombocytopenia.